The present invention relates to an optical information recording medium which allows information to be recorded, erased and reproduced by irradiation with light. Particularly, the present invention relates to a rewritable phase change type optical recording medium such as an optical disc, optical card or optical tape, which allows recorded information to be erased and rewritten and can record information signals at a high speed and a high density.
The conventional rewritable phase change type optical recording medium technology is described below. The conventional optical recording media have a recording layer mainly composed of tellurium, etc., and for recording, the recording layer in the crystalline state is irradiated with focused laser beam pulses for a short time, to be partially molten. The molten portions are quickly cooled and solidified by thermal diffusion, to form recorded marks of the amorphous state. The light reflectance of the recorded marks is lower than that of the crystalline state, and they can be optically reproduced as recorded signals. For erasing, the recorded marks are irradiated with a laser beam, to be heated to a temperature lower than the melting point and higher than the crystallization point of the recording layer, to crystallize the amorphous recorded marks, for restoring the original non-recorded state.
Known materials used as the recording layers of these rewritable phase change type optical recording media include alloys such as Ge2Sb2Te5 (N. Yamada et al., Proc. Int. Symp. on Optical Memory, 1987, p. 61-66). The optical media using these Te alloys in their recording layers have a high crystallization rate and allow high speed overwriting by one circular beam simply by modulating the irradiation power.
An optical recording medium using such a recording layer, for example, has a four-layer structure consisting of a first dielectric layer, a recording layer, a second dielectric layer and a metallic reflection layer provided in this order on a transparent substrate obtained by molding a polycarbonate resin. The dielectric layers act to prevent the recording layer form being deformed or opened during recording. Furthermore, the reflection layer acts to improve the signal contrast during reproduction by its optical interference effect.
Known materials used as the dielectric layers in contact with the recording layer include sulfides such as ZnS (Nishiuchi et al., Proceedings of 35th Joint Lecture Meeting of Physics, 28P-ZQ-3, P. 842 (1988)) and a mixture film of ZnS and SiO2 as a less internally stressed material (Toshimitsu Kurumizawa et al., 35th Joint Lecture Meeting of Physics, 28P-ZQ-3, P. 839 (1988)).
The above mentioned conventional rewritable phase change type optical recording media have the following problems.
If a layer containing a sulfide such as ZnS is used as a layer in contact with a recording layer, there arise such problems that since the erasability of the recording layer declines, it becomes difficult to secure sufficient erasability under recording conditions high linear speed and high density, and that the decreaseage of signal amplitude and the decline of erasability occur during repeated recording to increase the jitters of reproduced signals.
Furthermore, the conventional constitution has the following problems. First of all, if a recorded recording medium is allowed to stand for a long time (hereinafter called the archival property), it can happen that recorded marks vanish, and that the dielectric layers are separated from the recording layer, to cause bursting. Furthermore, if a optical recording medium which has signals recorded and has been allowed to stand for a long time has signals overwritten on a track having signals recorded (hereinafter called the overwrite shelf property), the jitters maybe increased to cause an error disadvantageously. So, the optical recording medium has a problem in storage durability.
For example, International Publication WO 96/17344 discloses a structure in which carbon is disposed to contact a recording layer. In more detail, the second dielectric layer contains carbon or a mixture with a metal or semimetal added to carbon. In this case, It is Intended to lessen the variations in the size of recorded marks by keeping the photoabsorption of the recording layer in the crystalline state larger than that in the amorphous state. However, this structure has a problem that it is insufficient in the above repetitive overwritability and storage durability.
Japanese Patent Laid-Open (Kokai) No. Hei3-100936 describes an optical recording medium having a substrate, a dielectric layer, a recording layer, a carbon layer and a dielectric layer laminated in this order. It is stated that the disc can be made higher in sensitivity since the carbon layer can absorb light. However, this structure has a disadvantage that the above problems of repetitive overwritability and storage durability cannot be solved.
Japanese Patent Laid-Open (Kokai) No. Hei2-139283 discloses an optical recording medium consisting of a substrate, a transparent layer (ZnS or ZnS-C), a carbon layer (10 nm or less), a recording layer (Ge2Sb2Te5), a carbon layer (10 nm or less) and a reflection layer. However, this structure has a disadvantage that the repetitive overwritability and the storage durability are insufficient in the case of high density edge recording.
An object of the present invention is to provide a rewritable phase change type optical recording medium less likely to have increased jitters, lower contrast, bursting, etc. respectively caused as deterioration by repeated overwriting. Another object of the present invention is to provide a rewritable phase change type recording medium excellent in storage durability such as archival property and overwrite shelf property.
The objects of the present invention can be achieved by an optical recording medium, which comprises at least a first dielectric layer, a first boundary layer In contact with a recording layer, and the recording layer laminated in this order on a substrate, wherein information can be recorded, erased and reproduced by irradiating the recording layer with light, and the recording and erasure of information are effected by reversible phase change between the amorphous phase and the crystalline phase of the recording layer, and said first boundary layer is mainly composed of at least one selected from
(1) oxides of elements (excluding carbon) belonging to group 3A through group 6B of the 2nd period through the 6th period In the periodic table,
(2) carbides of elements (excluding carbon) belonging to group 3A through group 6B of the 2nd to the 6th period In the periodic table,
(3) nitrides of elements (excluding carbon) belonging to group 3A through group 6B of the 2nd period through the 6th period In the periodic table, and
(4) carbon or compounds with carbon and oxygen and/or nitrogen, and said recording layer comprises the composition represented by the following formula (I):
{(Ge0.5Te0.5)x(Sb0.4Te0.6)1-x}1-y-zSbyAzxe2x80x83xe2x80x83(I) 
(where A represents one selected from the elements belonging to group 3A through group 6B of the 2nd period through the 6th period in the periodic table, excluding Ge, Sb and Te, and x, y and z satisfy the following relations:
0.2xe2x89xa6xxe2x89xa60.8, 0.01xe2x89xa6yxe2x89xa60.08, z=0, or
0.2xe2x89xa6xxe2x89xa60.8, 0xe2x89xa6yxe2x89xa60.08, 0 less than zxe2x89xa60.2)
Furthermore, the objects of the present invention can also be achieved by an optical recording medium, which comprises at least a first dielectric layer, a first boundary layer in contact with a recording layer, the recording layer, and a second boundary layer in contact with the recording layer, laminated in this order on a substrate,
wherein information can be recorded, erased and reproduced by irradiating the recording layer with light, and the recording and erasure of information are effected by reversible phase change between the amorphous phase and the crystalline phase of the recording layer, and
said first boundary layer and said second boundary layer are mainly composed of at least one selected from
(1) oxides of elements (excluding carbon) belonging to group 3A through group 6B of the 2nd period through the 6th period in the periodic table,
(2) carbides of elements (excluding carbon) belonging to group 3A through group 6B of the 2nd to the 6th period in the periodic table,
(3) nitrides of elements (excluding carbon) belonging to group 3A through group 6B of the 2nd period through the 6th period in the periodic table, and
(4) carbon or compounds with carbon and oxygen and/or nitrogen, and said recording layer comprises the composition represented by the following formula (II):
{(Ge0.5Te0.5)x(Sb0.4Te0.6)1-x}1-y-zSbyAzxe2x80x83xe2x80x83(II) 
(where A represents one selected from the elements belonging to group 3A through group 6B of the 2nd period through the 6th period in the periodic table, excluding Ge, Sb and Te, and x, y and z satisfy the following relations:
0.2xe2x89xa6xxe2x89xa60.95, 0.01xe2x89xa6yxe2x89xa60.08, z=0, or
0.2xe2x89xa6xxe2x89xa60.95, 0xe2x89xa6yxe2x89xa60.08, 0 less than zxe2x89xa60.2)